Method of unsticking material delivery apparatus

ABSTRACT

A method of unsticking a material delivery apparatus comprises automatically generating and applying to the material delivery apparatus a cyclical drive signal that causes at least two levels of torque to be applied to the material delivery apparatus.

BACKGROUND OF THE INVENTION

This invention relates generally to a method of unsticking a material delivery apparatus and more particularly, but not by way of limitation, to a method of changing a material delivery apparatus from a static friction state to a dynamic friction state.

A material delivery apparatus of the type referred to in this disclosure is used to move a material into a mixing unit where the material is mixed with one or more other materials into a mixture. The materials and mixture can be of any type. For example, a sand screw can be used to convey sand, referred to as proppant, into a fracturing fluid made at an oil or gas well.

When a particular amount or rate of material is to be conveyed, the material delivery apparatus is controlled either manually or automatically to obtain the desired delivery of material. At least such automatic control typically includes generating an electric control signal that causes the apparatus to operate as desired. A feedback signal is also typically used to ensure that the actual response of the material delivery apparatus to the control signal is what is desired. Use of a feedback signal creates a closed loop control system.

Although the electric control signal and the feedback signal enable automatic control to be implemented, they can also cause a runaway condition if the material delivery apparatus is stuck at start-up or becomes stuck during operation. That is, if the control signal is trying to drive the apparatus but the feedback signal indicates no operation, the automatic controller can presume that an increase in the control signal is needed. If the control signal is increased and the apparatus then becomes unstuck, the increased control signal will overdrive the apparatus whereby too much material is delivered.

The foregoing has been known to occur in prior automatic control systems with which manual methods of unsticking sand screws have been used. These manual methods include applying a vibrator to the sand screw tube to force the friction between the sand and the screw into a dynamic condition. This helps the screw to turn since the coefficient of dynamic friction is much less than the coefficient of static friction. Another method of manually unsticking a sand screw includes hammering on the tube of the screw. If all else fails, the sand screw hopper will have to be shovelled out.

A shortcoming of such manual methods is that when they are used with an automatic control system, the aforementioned runaway can result. Additionally, manual methods require a human operator to be present at the material delivery apparatus, which might present a dangerous situation. Furthermore, manual methods can be relatively slow, and they can damage the equipment.

A stuck material delivery apparatus also presents the shortcoming of failing to deliver material when it is needed, which can be highly detrimental. For example, if a sand screw is struck so that it does not deliver proppant into a fracturing fluid, and yet the deficient fluid is pumped into a well to create a fracture for enhancing oil or gas recovery, the created fracture may not be adequately propped open whereby it closes when the fracturing pressure is released. If such were to occur, the stuck sand screw would have caused material and money to be wasted because of the inadequate fracturing job. This could also prevent the well from being productive.

In view of these shortcomings, there is the need for an automatic method of unsticking a material delivery apparatus. Such a method should be capable of use with other control methods that automatically control the material delivery apparatus during normal operation so that runaway does not occur when the apparatus is automatically unstuck. Such a method should also be capable of use from and after start-up to ensure that material delivery is not stopped due to the apparatus becoming stuck.

SUMMARY OF THE INVENTION

The present invention overcomes the above-noted and other shortcomings of the prior art by providing a novel and improved method of unsticking a material delivery apparatus. The method can be used with other automatic control methods to prevent runaway when the apparatus is unstuck by the present invention. The method obtains unsticking without operator presence at the apparatus and without operator action (although in the preferred embodiment the operator can manually initiate the automatic unsticking method). The method can be used each time the apparatus is started to ensure "unstuck start-up. The present invention operates quickly so that there is no waiting for someone to go bang on the apparatus. The present invention operates without striking the apparatus with an implement that could damage the apparatus.

The method of unsticking a material delivery apparatus broadly comprises automatically generating and applying to the material delivery apparatus a cyclical drive signal including a first phase during which a first torque is applied to the material delivery apparatus and a second phase during which a second torque is applied to the material delivery apparatus.

The present invention can also be defined as a method of changing a material delivery apparatus from a static friction state to a dynamic friction state, comprising: (a) generating an electrical control signal having a predetermined magnitude selected in response to time; (b) applying, in response to communicating the electrical control signal to the material delivery apparatus, at least two torques to the material delivery apparatus; (c) sensing for operation of the material delivery apparatus; (d) sensing for the lapsing of a predetermined time period; and (e) repeating steps (a) through (d) until operation of the material delivery apparatus is sensed or lapsing of the predetermined time period is sensed.

The present invention can be more particularly defined as an improvement in a method of automatically controlling a sand screw using a generated primary control drive signal. The improvement comprises: incrementing a time counter in response to the lapse of predetermined time increments; monitoring for a primary control drive signal and determining that the primary control drive signal has been generated; determining whether the time counter has counted an elapsed time less than a predetermined time period; monitoring for rotation of the sand screw and determining that rotation has not occurred; computing an unstick drive signal in response to a predetermined relationship between the unstick drive signal and the elapsed time counted by the time counter; and communicating the unstick drive signal to control the sand screw therewith instead of controlling the sand screw with the generated primary control drive signal until determining that rotation of the sand screw has occurred or determining that the time counter has counted an elapsed time not less than the predetermined time period.

Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved method of unsticking a material delivery apparatus. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiment is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system in which the present invention can be used.

FIG. 2 is a block diagram of a specific implementation of the system shown in FIG. 1.

FIG. 3 is a flow chart of a computer program implementing the present invention within the system of FIG. 2.

FIG. 4 is a graphical representation of a predetermined relationship between an unstick control signal and an elapsed time of a timing counter used in the control represented in the flow chart of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Incorporated herein by reference are U.S. Pat. application Ser. No 08/084,055 to Stephenson et al , filed Jun. 29, 1993 (attorney docket number HS92.204A1); U.S. Pat. No. 4,916,631 to Crain et al.; U.S. Pat. No. 4,779,186 to Handke et al.; and U.S. Pat. No. 4,751,648 to Sears, III et al. These disclose particular apparatus and methods with which the present invention can be used or implemented, but the present invention is not limited by or to these particular disclosures.

In general, the method of the present invention is useful with a system of the type illustrated in FIG. 1. The general system includes a material delivery apparatus 2 that delivers material at a delivery characteristic (e.g., an operating speed) that can be sensed by a sensor 4. The material delivery apparatus 2 is operated in response to a control signal from a controller 6 that can respond to whether a valid or invalid feedback signal is received from the sensor 4. The system shown in FIG. 1 can be used to deliver liquid, solid or gaseous material.

Although the method of the present invention can be used with the general system shown in FIG. 1, the present invention will be further described with reference to the system shown in FIG. 2. The FIG. 2 system is a particular implementation of the general system shown in FIG. 1.

In the FIG. 2 implementation, the material delivery apparatus 2 includes a sand screw 8 driven by a hydraulic motor 10 that is operated by a hydraulic motor drive system 12. The sand screw 8 is of the type used to deliver sand, referred to as proppant, into a mixing unit 14 for mixing with a fracturing fluid at an oil or gas well site. The hydraulic motor 10 and the hydraulic motor drive system 12 are of conventional types used in such a fracturing fluid mixing system.

The hydraulic motor drive system 12 typically includes a pump that pumps hydraulic fluid to operate the hydraulic motor 10. The pump works in response to a prime mover such as an engine or motor. The hydraulic motor drive system 12 also includes a flow rate control device such as a swash plate within the pump if the pump is a variable displacement pump or an external electro-hydraulic valve, for example, if the pump is a pressure compensated hydraulic pump.

The sand screw 8 has an auger-like screw member that is rotated by the motor 10 to transport sand from an inlet of the sand screw 8 to an outlet communicating with the mixing unit 14. The rate of rotation of this screw element is sensed by a sand screw speed sensor 16. A conventional sensor 16 is typically calibrated to generate an electric signal representing the measured rotational speed of the sand screw 8 in revolutions per minute. An optical encoder is an example of one type of suitable sensor 16.

The signal from the sand screw speed sensor 16 is a feedback signal that is provided to the controller 6. The controller 6 also responds to a signal from a flow meter 18 indicating the flow rate of the fluid into the mixing unit 14. The controller 6 also responds to operator input. In response to such inputs, the controller 6 outputs a primary control signal to the hydraulic motor drive system 12 for controlling the rotational speed of the sand screw 8 and thereby controlling the flow of sand into the mixing unit 14. Through this control, a fracturing fluid with a desired concentration of sand is produced in the mixing unit 14.

The controller 6 can be any suitable type of automated process controller, whether analog or digital, discrete or integrated; however, it preferably includes a multi-tasking digital computer such as a Halliburton Services ARC Unit Controller.

The system described with reference to FIG. 2 is a conventional system used in the oil and gas industry. It is a closed loop system which preferably controls the sand screw 8 in response to the feedback provided by the speed sensor 16; however, it can operate in open loop control also, such as disclosed in the above-cited patent application.

The preferred embodiment of the present invention is implemented through the combination of the conventional equipment and new programming of the controller 6. This programming and the method of the present invention will be described with reference to FIGS. 3 and 4.

The method of the present invention as implemented by the equipment programmed in accordance with the flow chart of FIG. 3 generally comprises automatically generating and applying to the material delivery apparatus 2 a cyclical drive signal including a first phase during which a first torque is applied to the material delivery apparatus 2 and a second phase during which a second torque is applied to the apparatus.

The first torque referred to above is preferably maximum torque that can be applied to the material delivery apparatus 2. This maximum torque can, at least in some systems, be generated with a less than maximum control signal. The second torque referred to above is preferably a minimum torque, such as zero torque.

The respective torque is preferably applied for a relatively short time. In the preferred embodiment this short time is defined by the first phase of the drive signal having a first predetermined time period. For example, in a particular implementation, the first predetermined time period is preferably less than one second. The same applies for a second predetermined time period for which the second phase of the drive signal is preferably defined. It is contemplated that the phases of the drive signal can be variably defined, such as based on selected operating conditions (e.g., pump pressure); but in the preferred embodiment described herein the phases are defined for predetermined time periods.

The method of the preferred embodiment repeats the generation and application of the cyclical drive signal until either the appropriate operation of the material delivery apparatus 2 is sensed or a predetermined number of the cyclical signals have been generated and applied.

Specifically referring to FIG. 3 as applied to the sand screw system of FIG. 2, a software implemented time counter is initialized to a starting state. In FIG. 3, this counter is identified as "time passed" which is initially set to zero and then incremented every 0.01 second by a timer routine in a particular implementation. Thus, in the illustrated preferred embodiment, the time counter is incremented in response to the lapse of each 0.01 predetermined time increment.

FIG. 3 also shows that an unstick option time is predetermined within the program. In the preferred embodiment, the option time is predetermined as a selected time period; however, it is contemplated that other predetermining criteria could be used (e.g., an operating condition). An option time of 1.8 seconds is illustrated in FIG. 3. The cycle period of the unstick drive signal is preferably less than the unstick option time so that the option time defines the maximum number of cycles that the method will perform before it stops trying to unstick the sand screw 8. This is the automatic default time period that is used in a particular implementation of the preferred embodiment each time the sand screw is first started. Although not shown in FIG. 3, the program of the particular implementation also includes a time period of 4.2 seconds that is used the first time an operator manually activates this method by appropriately operating the controller 6 (e.g., by toggling between automatic and manual control of the main control program of a specific implementation disclosed in the co-pending patent application referred to above). The method can also be designed so that upon subsequent manual activation by the operator, one or more increased option time periods are established so that more attempts at unsticking the sand screw can be made during any one automatic unstick operation.

The method represented in FIG. 3 further includes monitoring for the primary control drive signal generated by the main control method and determining whether the primary control drive signal has been generated. If this control signal has not been generated (i.e., it is not greater than zero), the method of the present invention continually loops back to reset the "time passed" counter to zero and continue monitoring for the primary control drive signal. When the primary control drive signal is detected, the method of the present invention then determines whether the "time passed" counter contains a count representing a time less than the predetermined unstick option time. If the elapsed time is not less than the option time, the maximum number of cycles to be generated during that particular application of the present invention has occurred and the method loops back to again test for the primary drive signal as shown in FIG. 3. If the unstick option time has not been reached, the method of the present invention monitors for unstuck rotation of the sand screw and determines whether such rotation has occurred. If such rotation has occurred, the method of the present invention loops back to again monitor for the primary drive signal. If the screw speed is zero in the particular embodiment of FIG. 3, thereby indicating that the sand screw is stuck, the method of the present invention computes an unstick drive signal in response to a predetermined relationship between the unstick drive signal and the lapsed time counted by the "time passed" counter. Whether the sand screw speed is zero is determined by sensing the feedback signal provided by the sand screw speed sensor 16.

The unstick drive signal of the preferred embodiment is an electrical control signal generated by the controller 6 programmed in accordance with FIG. 3. This signal is generated by checking the elapsed time in the "time passed" counter and selecting the output value for the control signal from a predetermined relationship stored in the controller 6. A non-limiting example of such a relationship is illustrated in FIG. 4, which shows that a constant magnitude output is provided anytime during the respective first phase time periods from zero to 0.3 second, 0.6 to 0.9 second and 1.2-1.5 seconds. A zero level output is provided for the respective second phases of each cycle, namely from 0.3-0.6 second, 0.9-1.2 seconds and 1.5-1.8 seconds. This same relationship continues for longer unstuck option times. Other relationships can be used, and other criteria for defining or generating the unstick drive signal can be used.

In the preferred embodiment, the higher magnitude signal provided during the respective first phase of each cyclical drive signal is preferably large enough to cause the sand screw 8 to be driven with maximum torque for the very short duration of the respective first phase. In a particular implementation, this can be done by providing to the hydraulic motor drive system 12 an electrical control signal of between 40 percent and 70 percent of the maximum control signal that could be applied. In one implementation, this unstick drive signal is 50 percent of maximum and operates the swash plate of a variable displacement hydraulic pump to provide sufficient flow rate from the pump to the hydraulic motor 10 whereby the sand screw 8 is driven with maximum torque. During the second phase of a cycle of such unstick drive signal, the swash plate is destroked to reduce the flow rate so that pressure is released and zero torque is applied to the sand screw 8. Comparable control of a pressure compensated hydraulic pump can be obtained by operating the outlet valve disposed between the pump and the hydraulic motor 10. With a pressure compensated pump, the first phase of the unstick drive signal is preferably long enough to open the control valve sufficiently to pressurize the system to the pressure limit of the pump, whereupon maximum torque is applied to the sand screw 8. In response to the second phase of this unstick drive signal, the valve is closed and the pump's pressure control destrokes the pump, sufficient to maintain compensated pressure. Closure of the valve blocks flow to the motor so that zero torque is applied to the sand screw 8.

The unstick drive signal as computed by the controller 6 is output by overwriting the software register normally containing the primary control drive signal that has been generated by a concurrently running control program with which the present invention can be used. Thus, the method of the present invention communicates the unstick drive signal so that it, rather than the primary control drive signal, controls the sand screw during the unstick operation. This continues until it is determined that either rotation of the sand screw has occurred or the "time passed" counter has counted an elapsed time which is not less than the predetermined unstick option time period.

After overwriting/outputting occurs, the method of the present invention loops back to the point of monitoring for the primary control drive signal.

Using the method as just described, the sand screw 8 is automatically hydraulically jarred by the alternate short duration high and low torque applications made during each unstick drive signal cycle. In testing, this method has been used to unstick a sand screw which could not be unstuck with a vibrator or by hammering on the tube of the sand screw.

A program listing including a particular implementation of the flow chart of FIG. 3 is disclosed in the Appendix forming a part of this specification.

It is to be noted that although the aforementioned steps may have been sequentially described in the preferred implementation, multi-processing preferably occurs so that changes can occur at any time. For example, the main control method through which the primary control drive signal is generated is run concurrently with the method of the present invention; and the global values of the unstick drive signal, time passed and screw speed are also concurrently updated.

Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While a preferred embodiment of the invention has been described for the purpose of this disclosure, changes in the construction and arrangement of parts and the performance of steps can be made by those skilled in the art, which changes are encompassed within the spirit of this invention as defined by the appended claims. ##SPC1## 

What is claimed is:
 1. A method of changing a rotatable member of a material delivery apparatus from a static friction state to a dynamic friction state, comprising:receiving in a computer a signal indicating rotational speed of the rotatable member; providing in the computer a register for receiving a primary control drive signal generated by the computer under a primary control program to operate the rotatable member at a desired material delivery speed; communicating the contents of the register to control the operation of the rotatable member; and performing in the computer concurrently with the primary control program a program for unsticking a stuck rotatable member, including:determining in the computer whether a primary control drive signal has been generated by the primary control program; incrementing a time counter in the computer in response to the lapse of predetermined time increments; determining in the computer whether the time counter has counted an elapsed time less than a predetermined time period; determining in the computer, from the signal received therein indicating rotational speed of the rotatable member, whether the rotatable member is rotating; in response to determining in the computer that a primary control signal has been generated and that the elapsed time is less than the predetermined time period and that the rotatable member is not rotating, generating in the computer an unstick drive signal; and overwriting the primary control drive signal in the register with the unstick drive signal to control the rotatable member therewith instead of controlling the rotatable member with the primary control drive signal concurrently generated by the primary control program during operation of the program for unsticking a stuck rotatable member.
 2. A method of automatically controlling a sand screw for conveying proppant into a fracturing fluid at an oil or gas well, comprising:(a) receiving, in a computer connected at the oil or gas well for controlling the sand screw, a signal indicating rotational speed of the sand screw; (b) providing in the computer a register for receiving a primary control drive signal generated by the computer under a primary control program to operate the sand screw at a desired proppant delivery speed; (c) communicating the contents of the register to control the operation of the sand screw; and (d) performing in the computer concurrently with the primary control program, and commencing at start-up of the sand screw, a program for unsticking a stuck sand screw, including:(d1) determining in the computer whether a primary control drive signal has been generated by the primary control program; (d2) incrementing a time counter in the computer in response to the lapse of predetermined time increments; (d3) determining in the computer whether the time counter has counted an elapsed time less than a predetermined time period; (d4) determining in the computer, from the signal received therein indicating rotational speed of the sand screw, whether the sand screw is rotating; (d5) in response to determining in the computer that a primary control signal has been generated and that the elapsed time is less than the predetermined time period and that the sand screw is not rotating, generating in the computer an unstick drive signal; (d6) overwriting the primary control drive signal in the register with the unstick drive signal to control the sand screw therewith instead of controlling the sand screw with the primary control drive signal concurrently generated by the primary control program during operation of the program for unsticking a stuck sand screw; (d7) repeating steps (d1) through (d6) until determining that rotation of the sand screw has occurred or determining that the time counter has counted an elapsed time not less than the predetermined time period; and wherein the unstick drive signal is generated by selecting a predetermined magnitude from memory within the computer in response to the elapsed time in the time counter for each repetition of said steps (d1) through (d6).
 3. A method as defined in claim 2, wherein:the sand screw is driven by a hydraulic motor operated by a hydraulic motor drive system responsive to electrical signals communicated thereto in response to drive signals stored in the register of the computer; and one such electrical signal, which is responsive to an unstick drive signal overwritten into the register, has a magnitude within the range between 40 percent and 70 percent of a maximum signal that could be applied to the hydraulic motor drive system.
 4. A method as defined in claim 3, wherein the hydraulic motor drive system includes a variable displacement hydraulic pump having a swash plate operated by said one such electrical signal to provide flow rate from the pump to the hydraulic motor sufficient to drive the sand screw at maximum torque.
 5. A method as defined in claim 3, wherein the hydraulic drive system includes a pressure compensated hydraulic pump and an outlet valve responsive to said one such electrical signal so that maximum torque is applied to the sand screw in response thereto. 